
(2) Our basis sets are
a family of Dunning's correlation-consistent basis sets
(cc-pV\(x\) Z; \(x\)  = D, T, Q) [1]}, [2]}, [3]}
and their core-valence correlated variants
(cc-pCV\(x\) Z; \(x\)  = D, T, Q) [4]}.
The cc-pV\(x\) Z and cc-pCV\(x\) Z basis sets were originally
developed for correlated methods such as CCSD(T),
but have been found to be
appropriate even for DFT-B3LYP and FNDMC
in properly reproducing the dynamic electron correlation. [1]}
In particular, the polarisation functions included implicitly in cc-pV\(x\) Z
are essential for properly describing the Si-O-Si bond.
To describe the correlation effects more precisely,
the present study considers the dynamic correlation
between core and valence electrons by applying the cc-pCV\(x\) Z basis sets,
which can reproduce the core-valence correlation
by minimising the difference in the correlation energies
between all-electron and valence-only (using pseudopotentials)
calculations. [6]}
The number of basis functions entailed in each basis set
is shown in Table REF .
As shown, accounting for the core-valence correlation is
more expensive for increasingly complete orbital and polarisation functions,
with a quadruple zeta-level basis set
adding close to 130 more basis functions
to the standard correlation-consistent basis set.
The number of basis functions is expected
to largely correlate with the reliability of the calculations,
particularly for the core-valence variants pertaining to all-electron calculations.
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